The robustness of the buildings, standing for millennia, is due to the lime mortars made with vegetable extracts and inspired by the shells of molluscs or the spines of sea urchins.
Located in western Honduras near the border with the Guatemala, the Copán deposit is located. Considered the cradle of Maya civilization, Oxwitik (its Mayan name, meaning ‘three roots’) was a powerful city-state who ruled one of its most important kingdoms between the 4th and 9th centuries AD But, though a thousand and a half years have passed since then, many of its buildings and altarpieces have stood the test of time remarkably well, a phenomenon which has intrigued scientists for centuries. Now, a team of researchers from the University of Granada (UGR) has just revealed the secret: its builders have devised lime mortars to which they have added plant extracts. The results have just been published in the journal ‘Science Advances‘. ”Until now it was not known what the secret was that the monuments built by the ancient Mayan builders currently have a excellent state of conservationdespite having been exposed for more than a thousand years to a very aggressive tropical climate”, explains the lead author of this work, professor of the Department of Mineralogy and Petrology of the UGR Carlos Rodríguez-Navarro. Rodríguez-Navarro’s team is not the first to try to unravel the mystery. Other groups have done all kinds of tests, but no firm enough conclusions have been reached. Now, thanks to modern techniques such as transmission electron microscopy (TEM) and high-resolution X-ray diffraction using synchrotron radiation, UGR researchers have revealed the “secret recipe” of the Mayan builders.
The study of ancient texts has made it possible to precisely date the volcanic eruptions that triggered the Little Ice Age in Europe. Specifically, the ancient lime mortars and stuccos of Copán included organic compounds and a cement of calcite crystals (CaCO3) with nano- and mesostructural characteristics (structure of said crystals from the atomic and molecular scale down to the micrometre) similar to those of biominerals from calcite such as those used by mollusks to build their shells, where the particles are known to make cements harder and more plastic. Many ancient civilizations used practices such as heating, adding water or including natural ingredients in the limestone to pproduce more durable lime plasters, allowing them to survive for millennia. The ancient Maya independently developed a similar strategy around 1100 BC plant extracts to further enhance performance. But that’s not all: some of these practices are still used by the local descendants of those countries, who mix the lime with certain saps taken from the bark of the trees. To prove that both practices corresponded, the authors collected plaster and stucco samples from ancient Mayan ruins and compared them with others created in the style of the current premises, which inherited the tradition. “We prepared replicas of lime mortar dosed with polysaccharide-rich extracts from the bark of trees common in the Maya area, such as chukum (Havardia albicans) and jiote (Bursera simaruba) – spiega Rodríguez Navarro. Our analytical results demonstrate that the replicas have characteristics similar to those of ancient Mayan mortars and stuccos which contain organic compounds“. Furthermore, they demonstrated that both Maya and current materials exhibit “and calcite cement which includes intercrystalline and intracrystalline organic compounds (polysaccharides) which give the mortar matrix a marked plastic behavior and greater toughness and resistance to breakage, while increasing their resistance to chemical alteration, as they reduce their dissolution rate”. In other words, this calcite cement, similar to that of mollusc shells or sea urchin spines, makes the material enormously strong, even if it has had to withstand harsh external conditions. The authors point out that these practices could help create perfect materials for the conservation of historical and artistic heritage and modern, sustainable construction.